Photovoltaic devices convert light to electrical potential. Solar cells are common photovoltaic devices in widespread use for converting light energy to electrical potential. Present uses of solar cells are limited by the inefficiency of solar cells. Typical crystalline solar cells have an efficiency of ˜15% under 1.35 kW/m2 space illumination. The best commercial cells have an efficiency of 19%.
Solar cells provide small voltage potentials in small scale applications, such as MEMs devices. Usefulness is limited by the inefficiency, however. The low levels of efficiency also make solar cells difficult to implement in portable power sources for generation of electricity that might be used in any number of portable devices requiring a portable power source.
For large scale energy production, such as sources for electrical generation, the low efficiency of present solar cells makes their use cost prohibitive compared to traditional sources used to generate electricity. With direct sun photovoltaic (PV) power and current solar cell technology, costs far exceed the costs of generating electricity from fossil fuels. A goal of current research efforts is lowering the cost of producing electricity from sunlight from solar cells to make generation more comparable in cost to generation of power from fossil fuels.
The principal problem in commercial silicon (Si) solar cells is that less than 40% of sun light effectively couples to the Si semiconductor that forms the basis of commercial photovoltaic cells. A silicon solar cell converts sunlight energy into direct-current (DC) electrical energy by the photovoltaic effect. As sunlight impinges on the top surface of a silicon crystal film, the incident light energy is used in freeing electrons from silicon atoms, allowing them to wander inside the crystal. By special contact designs, those electrons are harnessed as an electric current for operation of an electronic instrument or appliance.
Other photovoltaic semiconductors include cadmium telluride (CdTe), and copper indium diselenide (CuInSe2). Semiconductors applied as thin films on a variety of inexpensive backings have been studied. Examples of PV products using thin film technology include photovoltaic roofing shingles and lightweight flexible panels used by backpackers and boaters.
Silicon-based PV is the most attractive material system because Si is the most abundant material and it is the least toxic of all. In addition, Si offers the opportunity for integration of solar power sources in silicon fabricated electronic devices, which remains the most widely used semiconductor fabrication material.
Silicon is inefficient as a photovoltaic converter. It has an indirect band gap material with a gap of 1.1 eV. Thus, long-wavelength infrared light of photon energy less than 1.1 eV (wavelength larger than 1.124 μm) does not have the threshold energy needed to free electrons from the Si atoms. In addition, deep red radiation converts mostly to heat in the cell. On the other hand, short wavelength light such as that in the ultraviolet part of the spectrum has more than enough energy to create electron hole pairs. The excess energy is transferred to the charge carriers and is dissipated as heat.
Crystalline silicon has a relatively low absorption coefficient, between 102 and 104 cm−1, which means that a thickness of about 200 μm is necessary to absorb most of the sunlight. However, efficient collection of the current is hampered by thick cells. The problem of providing sufficient light absorption in thin silicon has been an area of research. An example technique of texturing the surface of the solar cell has showed that cells that are as thin as 50 μm can be efficient.
Proposed techniques to increase efficiency of solar cells, in many cases, have had poor results. Despite many research efforts, a practical technique that is commercially feasible is still lacking. One Internet-posted study by Berkeley research students Becca Jones, Mike Scarpulla, Jessy Baker, Kevin Sivula, and Kirstin Alveri entitled “Nanocrystalline Luminescent Solar Converters”, C226 Photovoltaic Materials, Dated Dec. 6, 2004, describes study of techniques for coupling short wavelength light into a solar cell. The study describes the coating of a layer of CdSe/CdS core/shell nanorods onto a high-efficiency PV cell manufactured by Sunpower Corp to increase efficiency. No increase in efficiency was reported. The researchers recommended further research with conventional silicon PV cells with the nanorods embedded in transparent medium such as a glass or a transparent oxide or nitride layer.
United States Published Patent Application US2004/0126582, Jul. 1, 2004, discloses the use of an organic polymer to disperse nanoparticles, including silicon nanoparticles. The silicon nanoparticles are embedded in polymer matrix that is used to prevent aggregation of the particles or formation of closely-packed films. A solar cell is described with one or a series of polymer layers with dispersed nanoparticles therein, including layers having different sized luminescent silicon nanoparticles, with characteristic red, green and blue luminescent responses.
Others have proposed a porous silicon layer with its silicon nanostrucures as an active layer to be layered over a silicon solar cell. The use of porous layers, however, lacks control over the nanostructure distribution; it normally consists of a random and wide distribution in size and shape extending to sizes of tens of nanometers, which limits charge separation and collection or light propagation across the film. Also, porous layers are thick layers that might interfere with the operation of the underlying cell. At present, acceptable results are obtained for the use of porous Si as antireflecting coating for Si solar cells only. (See, e.g., “Porous silicon in solar cell structures: a review of achievements and modern directions of further use,” Yerokhov V. Y.; Melnyk I. I., Renewable and Sustainable Energy Reviews 3, 291-322(32), (1999); “Antireflective porous-silicon coatings for multicrystalline solar cells: the effects of chemical etching and rapid thermal processing, R J Martin-Palma, L Vázquez, J M Martinez-Duart, M Schnell and S Schaefer,” Semicond. Sci. Technol. 16 657-661 (2001); G. Kopitkovas, I. Mikulskas, K. Grigoras, I.imkienë, R. Tomaiûnas, “Solar cells with porous silicon: modification of surface recombination velocity,” Appl. Phys. A 73, 495-501 (2001).
In another proposed technique (see, e.g., V. {hacek over (S)}vr{hacek over (c)}ek, A. Slaoui, J.-C. Muller Thin Solid Films, 451-452, 384-388 (2004)) for improving solar cell efficiency, silicon nanocrystals were prepared ex situ (pulverizing of electrochemical etched porous silicon) and were embedded into a spin-on-glass antireflecting SiO2 based solution and then spun onto standard silicon solar cells. The Si-nc/SiO2 layer was intended to serve as a luminescence down-converter. The influence on solar cell performances (internal quantum efficiency, current-voltage characteristic) was investigated in an indirect manner in terms of destroying the nanoparticles by high temperature to see their effect. An increase in efficiency was reported. However, the obtained contribution from the silicon nanocrystal based system was also reported to be rather weak. This is likely due first to the fact that the fraction of incorporated nanomaterial is limited to a few percent. In other words, a problem with this approach is that the silicon nanoparticles are incorporated in a composite with glass, resulting in a low density of nanoparticles, ˜1% of the composite. Second, the size distribution of particles in the composite is wide since present synthesis techniques lack control over size or shape. Third, it is possible that matrix material is not smooth enough to provide optical confinement and index of refraction matching.